As imaging elements mounted to a digital video camera, complementary metal oxide semiconductor (CMOS) image sensors (hereinafter, simply referred to as the CISs) have been known. Among the CISs, there is a CIS that has a function called a global shutter in which simultaneity of an exposure time of an image to be imaged has been secured.
In addition, with the miniaturization of a semiconductor process, a voltage applied to a gate of an element such as a metal oxide semiconductor field effect transistor (MOSFET) has decreased.
For example, a relatively high voltage is applied to a gate provided in a pixel of the CMOS image sensor and initialization of each element in the pixel or signal transmission from a photoelectric converting unit to a charge/voltage converting unit in the pixel is performed. However, it is difficult to secure the voltage applied to the gate in the pixel, from the reason described above.
Therefore, a solid-state imaging element that applies a negative voltage to a well when signal transmission is performed to increase a potential level of a photoelectric converting unit and a potential level of a gate unit has been suggested (for example, refer to Patent Document 1).
That is, in such a solid-state imaging element, a photodiode PD21 and a floating diffusion FD22 are provided in a P well region W11 formed in a silicon substrate, as illustrated in FIG. 1.
In this example, the photodiode PD21 includes a P+ layer (charge isolation region) and an n layer (charge accumulation region) to accumulate charges. If a voltage is applied to a transfer gate unit GT23, the charges accumulated in the photodiode PD21 are transferred to the floating diffusion FD22 and are read as signal charges.
In addition, if a voltage is applied to a gate of a reset transistor RT24 according to necessity, the charges accumulated in the floating diffusion FD22 are discharged to the outside and a pixel is initialized.
In the solid-state imaging element illustrated in FIG. 1, when the charges are transferred from the photodiode PD21 to the floating diffusion FD22, the voltage is applied to the gate of the transfer gate unit GT23. However, if the voltage is insufficient, the transfer remainder of the charges is generated as shown by a polygonal line PO11 of the left side in the drawing.
The polygonal line PO11 shows a potential of each unit of the solid-state imaging element, that is, a potential at each position of the photodiode PD21, the transfer gate unit GT23, the floating diffusion FD22, and the reset transistor RT24. In FIG. 1, a downward direction is a positive direction of a potential.
In this case, a potential of a portion shown by an arrow A11, that is, a region right below the transfer gate unit GT23 becomes higher than a potential of a portion of the photodiode PD21. For this reason, a part of the charges of the photodiode PD21 may not be transferred to the floating diffusion FD22 and may remain in the photodiode PD21.
Therefore, in the solid-state imaging element, a negative voltage (negative bias) is applied to the P well region W11, so that the potential of the portion of the photodiode PD21 becomes higher than the potential of the region right below the transfer gate unit GT23, as shown by a polygonal line PO12 of the right side in the drawing.
That is, in a portion shown by an arrow A12 of the polygonal line PO12, if the negative voltage is applied to the P well region W11, the potential of the portion of the photodiode PD21 and the potential of the portion right below the transfer gate unit GT23 increase.
However, the potential of the portion of the photodiode PD21 becomes higher than the potential of the portion right below the transfer gate unit GT23 due to a sensitivity difference of these portions for the negative voltage and assistance to reading of the signal charges is performed. Thereby, a large amount of charges are transferred to the floating diffusion FD22.
As such, the negative bias is applied to the well region of the solid-state imaging element. For this reason, even when the sufficient voltage cannot be applied to the transfer gate unit in the pixel by making the voltage low, the assistance to the reading of the signal charges is performed and a dynamic range of a pixel signal can be expanded.